Electrochromism generally refers to a reversible change in optical properties of a material upon application of a potential. In particular, electrochromic materials exhibit a reversible color change due to an electrochemical reduction-oxidation (redox) reaction caused by application of an electric field. Electrochromic materials may include both organic and inorganic materials.
Electrochromic materials are commonly used in electrochromic devices. A typical electrochromic device includes a four layer assembly: (i) a first transparent electrically conductive film, (ii) an electrochromic material (organic or inorganic) deposited on the first conductive film, (iii) a second transparent electrically conductive film spaced apart from the first conductive film, and (iv) an ionic conductive medium (electrolyte) disposed between the electrochromic material and the second conductive film. It has been found that the inclusion of only one electrochromic film may result in electrochromic devices with shortened lives. Accordingly, a second electrochromic material film may be deposited between the ionic conductive medium and the second conductive film noted above to facilitate completion of the redox process in the device and reduce or eliminate degradative reactions in the electrolyte. This five layer assembly may be used to obtain two mixed colored states or, may, using two materials with complementary optical characteristics, enhance the contrast between the previously defined states.
Currently, forming the electrolyte layer with suitable materials remains a huge challenge. The development of electrochromic devices has been greatly restricted by the development of highly stable and transparent electrolytes. Previous electrochromic devices may use liquid/gel electrolytes or inorganic solid ion conductors. Liquid/gel electrolytes, most of which are organic solvent-based (e.g., carbonate, acetonitrile, etc.) electrolytes, have leakage of the liquid electrolyte, evaporation and exhaustion of the organic solvent, and potential safety issues. Inorganic solid electrolytes (e.g., LiPON, etc.) need high vacuum pressure to evaporate/sputter and thus often have high cost. In addition, inorganic solid electrolyte has poor mechanical flexibility.
There are several problems associated with known electrochromic devices and/or the components thereof. U.S. Pat. No. 6,667,825 ('825 patent) discloses an electrochromic device utilizing two conjugated polymer coated ITO-coated glass electrodes, and an ionic liquid such as [BMIM][BF4] as the electrolyte. The [BMIM][BF4] liquid electrolyte of the '825 patent does not include a Lewis acid, which results in improved stability and lifespan of the electrochromic device. Further, the electrochromic device of the '825 patent may avoid, at least in part, issues arising with residual images after quenching and electrolyte decomposition that are typically found in devices using organic solvent-based liquid electrolytes and ionic liquid electrolytes containing a Lewis acid. However, the electrochromic device of the '825 patent is still subject to problems associated with leakage of the liquid electrolyte, and the inability to be formed into thin films and film-shaped products.
In order to complement such disadvantages of liquid electrolytes, solid polymer electrolytes have appeared recently. S. A. Agnihotry discloses a polymer electrolyte having a high ionic conductivity of 10−3 S/cm at room temperature, the polymer electrolyte being formed by adding a small amount of PMMA (polymethyl methacrylate) polymer and fumed silica to an electrolyte formed of propylene carbonate containing 1M LiCIO4 added thereto (see, Electrochimica Acta, 2004, 49: 2343-2349). However, because the above polymer electrolyte still uses an organic solvent as electrolyte, this device still has several disadvantages such as low quenching rate, residual images after quenching, decomposition and exhaustion of organic solvent-based electrolytes, or the like. In addition, because of the swollen states of the above polymers, the interfaces and the phases of the above polymers are relatively unstable. Over time, the interfaces of the above polymers may change easily, leading to the failure of electrochromic devices.
Accordingly, there is a need in the art for an improved solid polymer electrolyte suitable for use in electrochromic devices.